In manufacturing systems it is of course desirable to achieve production rates which are as high as possible. However, in situations where consecutive operations are to be performed on the workpieces at separate independent machines (stations) requiring the fixturing or re-locating of the workpiece for each consecutive operation each having different station cycle times, it is difficult to achieve and maintain desired high production and efficiency rates. Prior attempts have normally centered around providng a limited number of redundant stations for the stations with long cycle times, however there are economic and severe technological limits on the number of redundant stations that can be utilized to achieve desired high production rates with an integrated system. Also, in prior rate limited systems when breakdowns or station interruptions occur--especially at slower stations--the entire manufacturing operation can be slowed down or brought to a halt.
According to the present invention an integral dual transport assembly, and a method of utilizing same with independent machines or stations, are provided which eliminate most of the problems associaed with prior art conveyorized manufacturing systems or transfer type machines, and permits a predetermined high production rate to be achieved and maintained. In conventional systems redundant stations normally become impractical generally after three independent redundant stations due to added transfer time and requirements to identify processed and to be processed workpieces. However, according to the invention 20 or more redundant stations are easily accommodaed and may be productively balanced with other groupings of redundant or single stations as the operations and station cycle times dictate to achieve desired production rates and system reliability.
According to the present invention, workpieces are mounted on workpiece transport fixtures (hereinafter called "pallets") which pallets have random access to a plurality of common stations with no intermingling of processing, processed, and waiting-to-be processed parts within a common station grouping. There is no need to return through the system prior to reaching the next downstream operation, and maximum flow for the to-be-processed workpieces is achieved, greatly simplifying the memory and/or control system. While not limiting the number of redundant stations as in other systems due to pallet transfer times or the co-mingling of processed and to-be-processed parts.
The assembly and method according to the present invention operate according to an accumulation-in-motion theory; i.e., in the majority of instances pallet accumulation will be live, transferring down the in-feed conveyor in search of an empty station and automatically being directed into such first available station immediately upon arrival at the station. However, should the pallet arrive adjacent to the last station in the series and find it occupied it will simply wait adjacent to the station until that station is available. The pallets may be proportionally retained adjacent to each of the stations rather than being allowed to accumulate at the end station, thus as each station is occupied from the last station in line on back, individual pallets are queued adjacent to a station rather than queuing at the last available station. When the last station or an upstream station accepts the adjacent pallet the other pallets held adjacent to the preceding stations are released, and thus the system has the ability to vary the pallets in static float. This unique feature serves to balance the system and reduce the number of pallets that would otherwise be required, although by the very nature of this system float will be dynamic with static flow coming into play only when normal systems balance has been disturbed or a given series of redundant stations lack the production capability of the other elements making up the system and hence becomes the slowest and controlling operation within the system (e.g., where one or more stations of the series of redundant stations breaks down or functions at reduced efficiency).
The system according to the present invention provides for the conveyance of a plurality of pallets containing workpieces to and between work stations. The system comprises first and second conveyors adapted to workpiece transport and locating pallets thereon, means for mounting the conveyors so that they are generally parallel to and spaced from each other over the working portions thereof, and means for powering the conveyors so that they run in the same direction, workpiece transport and locating pallels supported by the first conveyor running generally parallel to pallets supported by the second conveyor. Pallet locating means integrated with workpiece acting means are generally disposed between and extend between the first and second conveyor for locating the pallet and workpiece and acting upon workpieces brought into contact therewith and means are provided for moving workpiece locating and supporting pallets from the first conveyor to a said workpiece acting means and then to the second conveyor, or vice-versa. The conveyors preferably comprise flat-top plate chain conveyors, although a wide variety of conventional conveyor types may be used. Preferably, pallet stop means are associated with each conveyor on either side of the workpiece acting means. Also pallet locating and clamping means are provided for clamping and locating a pallet in operative association with the workpiece acting means, fluid powered actuating means are provided for effecting actuation of the stop means, clamping means, and pallet moving means; electrically controlled valve means are provided for controlling the fluid powered actuating means; and electric sensing and control means are provided for effecting actuation of the electrically controlled valve means in response to pallet position and condition. Status indicators of any conventional type may be provided on each pallet, including mechanical indicators (movable pins, levers, etc.), magnetic, optical, or electric devices.
According to the method of the present invention it is possible to achieve and maintain a desired predetermined manufacturing system rate utilizing first and second generally parallel conveyors adapted to transport pallets containing workpieces thereon in the same direction, with workpiece acting stations normally disposed between the conveyors. In practicing the method, a plurality of workpiece acting stations are disposed in sets, all stations in each set for performing a given task on the workpieces. The number of stations provided for each set is proportional to the relative amount of time that it takes to do the particular task performed by the stations. Pallets containing workpieces thereon are fed on the first conveyor toward the first set of stations, and each pallet is selectively moved, depending upon station availability, into operative association with a station of the first set by moving the respective pallet off of the first conveyor toward the station. After completion of the desired activity upon each workpiece at a station in the first set, the pallet is moved from the station onto the second conveyor and then transported on the second conveyor toward a second station set. Each pallet is selectively moved into operative association with a station of the second set, depending upon station availability, by moving the respective pallet off of the second conveyor toward the station. After completion of the desired activity upon each workpiece at a station in the second set the respective pallet is moved from the station onto the first conveyor, without regard to the completion of operations at the other stations within the system. A nonconvergent traffic control pallet stop permitting egress of the pallet from the station onto the conveyor may be actuated. Then the sequence of steps is repeated for each station set until all desired operations have been completed for a given pallet mounted workpiece. All of the steps are preferably accomplished automatically, although manual work stations can be interspersed with automatic work stations, and automatic stations (as permitted by cycle requirements) may be on-line with either of the two conveyors.
The condition of each pallet and workpiece is automatically sensed at a plurality of different points for a plurality of different conditions, and upon sensing of a given condition a pallet and/or workpiece on or thereon may be selectively rejected by removing them from the first and second conveyors or simply passed through the system without the performance of additional work. Completed workpieces may also be automatically removed from the conveyors. Under normal operating conditions pallet accumulation at the stations within a set is live, and during abnormal operating conditions the pallets are varied and quyed in static float behind each station of a set of stations. Normally idle back-up work stations may be provided in at least some of the station sets, and brought into operation should a corresponding work station become inoperative to assure maximum efficiency and through put concurrent with a maintenance procedure.
It is the primary object of the present invention to provide a transport assembly for, and method of, achieving and maintaining a desired predetermined manufacturing system production and efficiency rate, which can readily achieve high production capabilities in a simple manner while accommodating a variety of widely differing production and manufacturing requirements. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.